The present invention relates to the regulation of electrolytic cells, and more particularly, to methods for the automatic adjustment of the interpolar distance between the anodes and the cathodes used for electrolysis of alkali metal and alkaline earth metal chlorides with a mercury cathode functioning at high current densities to produce chlorine and alkaline hydroxides.
In each cell the anodic assembly is comprised of plural elements which are mechanically carried by beams, the height of which beams can be adjusted. Generally, each anode element is dimensionally stable or is itself individually adjustable in height in its connection to the beam. Thus, from an electrical standpoint, the anodic elements are generally associated in groups, each group being fed energy by a special conductor called a current supply or feed bar.
The goal of the makers and users of chlorine plants at all times is to utilize a minimum amount of power for each ton of chlorine produced in the cells and to operate with the highest possible current density. In order to attain this goal, it is necessary to control very precisely the electrolysis voltage. A consideration of the factors going into the required electrolysis voltage shows that it is composed of the following terms for each anodic element:
A. The voltage drop in the metal supporting the anode. PA1 B. The voltage drop at the contact point of the metal support with the anode. PA1 C. The voltage drop in the anode itself. PA1 D. The thermodynamic oxidation potential of the chloride ion. PA1 E. The anodic overvoltage. PA1 F. The voltage drop in the electrolyte. PA1 G. The thermodynamic reduction potential of the alkali or alkaline earth metal at the mercury cathode. PA1 H. The cathodic overvoltage, and PA1 I. The voltage drop in the mercury and cathodic steel work.
Of the foregoing terms, (a), (c), and (i) are determined by the fabrication of the cell; the terms (d), (e), (g), and (h) are characterized by the particular electrochemical processes used; (b) is determined by the quality of the anode wiring; and (f) is determined by the interpolar distance between the anodes and cathode and by the electrolyte temperature.
When, instead of considering each individual factor, the entire cell assembly is considered, two additional factors must be taken into account:
First of all, the mercury cathode layer is not even but is an irregular surface, and secondly, since it is formed of several elements the anode is not plane. It is therefore simplistic to speak of a single interpolar distance, and on the contrary, a great number of individual distances between the different anodic elements and the portion of the mercury layer situated immediately beneath each element must be considered to be scattered about an average interpolar distance.
As a result of this, the average electrolysis voltage is a complex function depending upon the construction technology of the cell, the thermal gradient along the cell, the current strength and density, the temperature and concentration of electrolyte, the mercury amalgam concentration in the cathode, and the mean interpolar distance. Among all these factors, the interpolar distance has a strong influence on the electrolysis voltage, and it is equally the factor which is the most easily altered. Thus, to optimize the mean electrolysis voltage, that is to say, the specific energy consumption, it is sufficient to control the mean interpolar distance, but this must be done carefully.
Experience has indeed shown that in order to get the cell to function at optimum conditions of voltage at a given operating intensity, it is necessary that the cell be at electrical equilibrium and then that the mean interpolar distance be as small as possible, taking into consideration the irregularities in shape of the cathodic layer and the anode assembly. Starting with this state of the art, it has been verified that the operation deteriorates over a period of time even with dimensionally stable anodes. If the cell is left to run by itself, short circuits occur after a period of time and the anode elements are damaged. To be able to control this, it is necessary to detect the short circuit condition promptly and then to move the anode-carrying beam holding the elements involved and thereby adjust the elements in order to get back to the optimum voltage.
If these operations were all to be executed manually, it would be expensive and slow and would moreover risk a delay in response after a short circuit is detected.
There is accordingly a commercial need to carry out the process automatically, so as to permit in the shortest possible time, on the one hand, information to be obtained on the electrical situation of the cell, and on the other hand to detect and arrest the short circuits before they become destructive. Finally, there is a need automatically to regulate the interpolar distance in the electrolysis cells so that the specific energy consumption of the cells is optimized. This need is considerably more acute when high current densities are used in the anodes.
Various methods have been suggested to deal with these problems. It is well-known that the interpolar distance can generally be controlled by mechanical adjustment of the anode height in relation to the cathode. Generally, an electric or a hydraulic motor is used, and this raises the possibility that the anode height can be raised or lowered in relation to the surface of the mercury layer. Judicious selection of the control parameter and the processing of the parameter to control the motor are the basic elements of an automatic control system for the interpolar distance.
It has been proposed for example to measure the voltage variations or the rate of change of voltage variations in the conductors carrying anode current. This parameter generally does not afford the required sensitivity efficiently to protect the anode assembly and permit control, particularly with dimensionally stable metallic anodes.